As it is represented by mobile base stations, signal modulation is becoming more sophisticated in a purpose of improving a transmission rate. As a result, an amplifier is required with a high distortion characteristic, and a need is arising to make the amplifier work with a back-off bigger than a saturated output state. Therefore, a higher output is required to the amplifier. As a solution, a method of overlapping electric powers, such as a push-pull type amplifier, is used.
FIG. 1 is a block diagram showing a configuration of a push-pull type high frequency power amplifier by a related art. In FIG. 1, the power amplifier of the related art includes a first marchant balun 100 as an input side balun circuit, a first transistor 200, a second transistor 300 and a second marchant balun 400 as an output side balun circuit. The first marchant balun 100 includes an RF (Radio Frequency) signal input section 110 as an input section, a ground end section grounded to a ground 120 and a first and a second output sections. The two transistors 200 and 300 have a same characteristic. The second marchant balun 400 includes a first and a second input sections, an RF signal output section 430 as an output section and a ground end section grounded to a ground 440. The first output section of the first marchant balun 100 is connected to a gate of the first transistor 200. One of a source or a drain of the first transistor 200 is connected to the first input section of the second marchant balun 400. The other of the source or the drain of the first transistor 200 is grounded. The second output section of the first marchant balun 100 is connected to a gate of the second transistor 300. One of a source or a drain of the second transistor 300 is connected to the second input section of the second marchant balun 400. The other of the source or the drain of the second transistor 300 is grounded.
FIG. 2 is a schematic diagram showing a port of an output side balun circuit of the related art in FIG. 1. This is the marchant balun 400 as an output side balun circuit and includes a first port 410 as a first input section, a second port 420 as a second input section, a third port 430 which is shown in FIG. 1 as the RF signal output section 430 and a ground end section grounded to the ground 440. For example, in a case in which this marchant balun 400 is used as the output side balun circuit, a signal outputted by the first transistor 200 to the marchant balun 400 is provided to the first port 410 to be transmitted to the third port 430. Similarly, a signal outputted by the second transistor 300 to the marchant balun 400 is provided to the second port 420 to be transmitted to the third port 430.
Here, a distance between the first port 410 and the third port 430 is to be set half a wavelength of a fundamental wave of the inputted signal longer than a distance between the second port 420 and the third port 430. In this way, it is possible to overlap signals outputted by the first transistor 200 and the second transistor 300 and having π radians of phase difference, with no loss.
However, here, second harmonics, which are generated by the two transistors 200 and 300, have a same phase.
A case in which a usual microstrip line is used as an output side balun circuit will be considered. Here, for a second harmonic, a difference between a distance from the first port 410 to the third port 430 and a distance from the second port 420 to the third port 430 corresponds to a zero phase difference. Consequently, in such balun circuit, the second harmonics are perfectly overlapped.
Also, for a third harmonic, a difference between the distance from the first port 410 to the third port 430 and the distance from the second port 420 to the third port 430 is 1.5 wave lengths long, and it corresponds to π radians of phase difference. Consequently, the third harmonics, which are outputted by the first transistor 200 and the second transistor 300 with a π radians phase difference, similarly to the fundamental wave, is overlapped in almost same phase. That is, in such balun circuit, there is no effect of cancelling the third harmonics.
Next, a case in which the marchant balun 400 is used as an output side balun circuit will be considered. Here, for the second harmonic, the distance from the first port 410 to the third port 430 becomes almost ½ wave length longer than the distance from the second port 420 to the third port 430. This makes a cancelling effect acting to the second harmonics and a distortion reduction effect is obtained.
However, here, the third harmonics are overlapped in almost same phase, similarly to the fundamental wave. For this reason, the cancelling effect for the third harmonic is very small.
As explained above, in a case in which a balun circuit is prepared by controlling a length of an ordinary transmission line and a difference of ½ wave length in a fundamental frequency is realized, the cancelling effect is not obtained for neither the second harmonic nor the third harmonic. Also, in a case in which a balun circuit such as the marchant balun is used, the difference becomes almost ½ wave length for the second harmonic, and a certain level of a cancelling effect for the second harmonic is obtained. However, due to a frequency dependence of the difference of balun circuit length, a perfect cancelling effect can not obtained. In addition, there is almost no cancelling effect for the third harmonic.
That is, it was difficult with the above mentioned balun circuit of the related art to realize a difference of ½ wave length for the fundamental wave, a difference of ½ wave length for the second harmonic and no difference of wave length for the third harmonic at same time. For this reason, the distortion characteristic of the amplifier was bad, and, to obtain a desired distortion characteristic of a system, there were a need to add a supplementary distortion compensation circuit and a problem that the amplifier becomes bigger.
Relating to the above, a mention about a power amplifier is disclosed in patent literature 1 (Japanese Laid-Open Application 2005-39799). The power amplifier mentioned in the patent literature 1 is to amplify a high frequency signal. This power amplifier includes a first amplifying device, a second amplifying device, a first distributed transmission line, a first resonating circuit and an output end. Here, the first amplifying device is to amplify a first signal. The second amplifying device is connected to the first amplifying device in a push-pull style and is to amplify a second signal having a phase opposite to the first signal. The first distributed constant line has a line length which inverts a phase of the fundamental wave component of the first signal amplified by the first amplifying device. The first resonating circuit is connected between a position on the first distributed constant line where a phase of a component of an even-order harmonic to be short-circuited is inverted and an output side of the second amplifying device, and is to resonate in series in a frequency of the component of the even-order harmonic to be short-circuited. The output end is to output after overlapping a signal from the first distributed constant line and a signal from the second amplifying device.